Curable, or thermosettable, coating compositions are widely used in the coatings art, particularly for topcoats in the automotive and industrial coatings industry. Basecoat-clearcoat composite coatings are topcoats that offer exceptional gloss, depth of color, distinctness of image, or special metallic effects. The automotive industry has made extensive use of basecoat-clearcoat composite coatings for automotive body panels. Single-layer topcoats and the clearcoats of color-plus-clear composite coatings require an extremely high degree of clarity and gloss to achieve the desired visual effect. Such coatings require a low degree of visual aberrations at the surface of the coating in order to achieve the desired high gloss and high distinctness of image (DOI).
Because defects are so noticeable in the smooth, glossy surfaces required for these coatings, they are especially susceptible to a phenomenon known as environmental etch. "Environmental etch" is a term applied to a kind of exposure degradation that is characterized by spots or marks on or in the finish of the coating that often cannot be rubbed out. It has been difficult to predict the degree of resistance to environmental etch that a high gloss topcoat or color-plus-clear composite coating will exhibit. Many coating compositions known for their durability and/or weatherability when used in exterior paints, such as traditional high-solids enamels in which a hydroxyl-function acrylic polymer is crosslinked with a melamine-formaldehyde resin, do not provide the desired level of resistance to environmental etch when used in high gloss coatings such as the clearcoat of a color-plus-clear composite coating. While the ether linkages formed by aminoplast resin crosslinkers like the melamine-formaldehyde resin with hydroxyl-functional resins are undesirable from the standpoint of resistance to environmental etch, aminoplast crosslinkers are desirable for other reasons such as providing high solids coatings having excellent appearance that cure under moderate conditions.
Recently, automotive coating compositions based on carbamate-functional acrylic resins have been shown to be more resistant to environmental etch damage. Curable coating compositions utilizing carbamate-functional acrylic resins are described, for example, in U.S. Pat. Nos. 5,356,669, 5,474,811, 5,552,497, 5,559,195, 5,605,965, 5,639,554, 5,639,828, and 5,726,246, each of by reference. Carbamate groups can be crosslinked at moderate temperatures with melamine-formaldehyde resins or other aminoplast resins. The coating compositions can thus take advantage of many of the benefits of aminoplast resins while at the same time forming coatings that are resistant to environmental etch degradation.
U.S. Pat. No. 5,356,669, for example, describes methods of preparing an acrylic polymer having carbamate functionality. One suggested method is to react a cyclic carbonate-functional acrylic with ammonia to form the carbamate group or groups. A carbonate-functional monomer could similarly be converted to a carbamate-functional monomer and then carbamate-functional monomer in turn used in the polymerization of the carbamate-functional acrylic polymer. These synthetic routes to producing the carbamate functionality have advantages. A drawback to the carbonate ring-opening synthesis is that the reaction also produces a hydroxyl group that may then react with an aminoplast crosslinker (as in the traditional enamel compositions) to form undesirable ether linkages.
Other methods that have been suggested for preparing a carbamate-functional acrylic polymer also result or potentially result in an acrylic polymer that has hydroxyl functionality in addition to the desired carbamate functionality. U.S. Pat. No. 5,553,497 describes a method of transesterification of a hydroxy-functional acrylic with a carbamate compound to produce a carbamate-functional acrylic polymer. In this case, again, the acrylic polymer may have hydroxyl groups in addition to the carbamate groups, as some of the hydroxyl groups would be expected to remain unconverted. In a different synthesis, U.S. Patent No. 5,552,497 discloses a simultaneous addition polymerization of hydroxy functional acrylic monomer and esterification with an alkyl carbamate such as methyl carbamate to produce a carbamate functional acrylic polymer. The simultaneous polymerization and esterification avoids problems of incompatibility of carbamate-containing monomer in the polymerization mixture and undesirable molecular weight gains during transesterification of a pre-formed hydroxyl-functional acrylic polymer. The acrylic polymer may have hydroxyl groups, however, if the esterification reaction is not complete. Another method of producing carbamate functionality is to react a hydroxyl group with HNCO, produced by the thermal decomposition of urea, as is described in U.S. Pat. No. 5,605,965. Hydroxyl functionality remains on the acrylic polymer when less than a stoichiometric amount of the HNCO is reacted.
While each of the methods just described is feasible for producing acrylic polymers having carbamate groups, residual hydroxyl groups may produce undesirable ether linkages with aminoplast crosslinking agents. It would thus be desirable to render the hydroxyl functionality incapable of reacting with the aminoplast crosslinker so that only the carbamate groups form crosslinks during curing of the acrylic resin in order to maximize resistance of the cured coating to environmental etch.